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How & Why Do Leaves Change Color?

Despite their astonishing record of losses when dealing with lumberjacks and beavers, trees are pretty tough customers. Their trunks, branches, roots and twigs are all more than capable of enduring a winter's worth of freezing temperatures, snow, sleet and hail. Their leaves, though? Eh, not so tough. The broad, thin leaves of a broadleaf tree (like a maple, an oak, a birch, or a poplar) are an Achilles' heel when winter comes, and are vulnerable to freezing and damage from the elements. In order to survive, the trees either have to somehow protect the delicate leaves or shed them.

Evergreen trees—your pines, spruces, firs, etc.— went the protection route. Their leaves, or needles, are covered in a waxy coating to resist freezing, allowing them to live for years or even decades before falling off and being replaced. The leaves of deciduous trees, on the other hand, are cast off with the arrival of winter. The chemical processes that prepare them for their send-off also treat us to the season's vibrant colors.

Color Coding

Green: The green color of leaves throughout spring and summer comes from chlorophyll, a pigment vital to photosynthesis.

As we get closer to autumn and some parts of the planet get fewer hours of sunlight, trees respond by stopping the food-making photosynthesis process and slowing the production of chlorophyll until, eventually, they stop producing it altogether and the green color of the leaf fades

leaves-mapleYellow and Orange: Along with chlorophyll, there are yellow and orange pigments, carotene and xanthophyll, inside some trees' leaves. For most of the year, these pigments are masked by chlorophyll, but as the chlorophyll breaks down and the green color dissipates, the yellow to orange colors become visible.

Red: Another class of pigment that occurs in leaves is the anthocyanins. Anthocyanins, unlike carotene and xanthophyll, are not present in leaves year-round. It isn't until the chlorophyll begins breaking down that the plant begins to synthesize anthocyanin. Why do trees begin producing a different pigment in leaves they're getting ready to lose? The prevailing theory is that anthocyanins protect leaves from sun damage, lower their freezing point, allow them to remain on the tree longer, and buy the tree more time to recover nutrients from its leaves. The colors that anthocyanins produce are dependent on the pH of the leaves' cell sap. Very acidic sap results in a bright red color, while less acidic sap leads to a purplish red.

Brown: The humdrum color is the result of waste products trapped in the leaves.

That covers the basics of how each of the colors can be produced. But which color we ultimately see depends on several factors, such as"¦

Species: Certain colors are characteristic of particular tree species and can be used to help identify the type of tree you're looking at. Oak leaves turn red, brown, or russet, hickories turn golden bronze, poplars turn golden yellow, dogwoods turn a purplish red, beeches turn a light yellow/tan, birches turn bright yellow, sugar maples turn orange-red, black maples turn a glowing yellow, and red maples turn scarlet. Some trees, notably elms, don't go through much color change at all; there's just a dull brown and then the leaf is gone with the wind.

Weather: The temperature and moisture levels a tree is exposed to before and during the time its leaves' chlorophyll breaks down can affect color. Sunny days and cool nights favor anthocyanin production and bright red leaves. On cloudy days, anthocyanin isn't as chemically active and allows the orange or yellow pigments to take center stage.

Geography: Autumn leaves in Europe tend to be mostly yellow, but the US and East Asia seem to favor red leaves. Scientists from Israel and Finland recently put forth a theory about this color difference in the journal New Phytologist1. The scientists think that some 35 million years ago—amid a series of ice ages—many tree species evolved to become deciduous and produced red leaves to ward off insects. In North America and Asia, north-to-south mountain chains enabled the north and south spread of plants and animals corresponding with the advance and retreat of ice. In Europe, east-to-west mountain ranges like the Alps trapped plant and animal life. Many tree species (and the insects that depended on them) died out when the ice advanced. At the end of repeated ice ages, say the scientists, the tree species that survived didn't need red leaves to cope with the insects that were left, so they stopped producing red pigments and stuck with yellow.

The Dead Leaves and the Dirty Ground

leaves-ground

While all this color changing and autumn magic is going on, the tree is preparing to cast off its leaves. Around the same time that chlorophyll production slows down, the veins that transport nutrients and water to the leaf from the rest of the tree get closed off. A layer of cells at the base of the leaf stem, called the separation layer, swells and forms a cork-like material, gradually severing the tissue that connects the leaf to the branch. The leaf falls off and the tree seals the cut—so when the leaf is blown off or falls from its own weight, a leaf scar is left behind.

1Lev-Yadun, S and Holopainen, J. (2009). Why red-dominated autumn leaves in America and yellow-dominated autumn leaves in Northern Europe? New Phytologist Volume 183(3): 506-512. doi:10.1111/j.1469-8137.2009.02904.x
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Special thanks to Damian Dockery, who provided the foliage photos. See more of his work at flickr.com/damiand23.

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Big Questions
What Does the Sergeant at Arms Do?
House Sergeant at Arms Paul Irving and Donald Trump arrive for a meeting with the House Republican conference.
House Sergeant at Arms Paul Irving and Donald Trump arrive for a meeting with the House Republican conference.
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In 1981, shortly after Howard Liebengood was elected the 27th Sergeant at Arms of the United States Senate, he realized he had no idea how to address incoming president-elect Ronald Reagan on a visit. “The thought struck me that I didn't know what to call the President-elect,'' Liebengood told The New York Times in November of that year. ''Do you call him 'President-elect,' 'Governor,' or what?” (He went with “Sir.”)

It would not be the first—or last—time someone wondered what, exactly, a Sergeant at Arms (SAA) should be doing. Both the House and the Senate have their own Sergeant at Arms, and their visibility is highest during the State of the Union address. For Donald Trump’s State of the Union on January 30, the 40th Senate SAA, Frank Larkin, will escort the senators to the House Chamber, while the 36th House of Representatives SAA, Paul Irving, will introduce the president (“Mister [or Madam] Speaker, the President of the United States!”). But the job's responsibilities extend far beyond being an emcee.

The Sergeants at Arms are also their respective houses’ chief law enforcement officers. Obliging law enforcement duties means supervising their respective wings of the Capitol and making sure security is tight. The SAA has the authority to find and retrieve errant senators and representatives, to arrest or detain anyone causing disruptions (even for crimes such as bribing representatives), and to control who accesses chambers.

In a sense, they act as the government’s bouncers.

Sergeant at Arms Frank Larkin escorts China's president Xi Jinping
Senat Sergeant at Arms Frank Larkin (L) escorts China's president Xi Jinping during a visit to Capitol Hill.
Astrid Riecken, Getty Images

This is not a ceremonial task. In 1988, Senate SAA Henry Giugni led a posse of Capitol police to find, arrest, and corral Republicans missing for a Senate vote. One of them, Republican Senator Bob Packwood of Oregon, had to be carried to the Senate floor to break the filibustering over a vote on senatorial campaign finance reform.

While manhandling wayward politicians sounds fun, it’s more likely the SAAs will be spending their time on administrative tasks. As protocol officer, visits to Congress by the president or other dignitaries have to be coordinated and escorts provided; as executive officer, they provide assistance to their houses of Congress, with the Senate SAA assisting Senate offices with computers, furniture, mail processing, and other logistical support. The two SAAs also alternate serving as chairman of the Capitol Police board.

Perhaps a better question than asking what they do is pondering how they have time to do it all.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Big Questions
What Makes a Cat's Tail Puff Up When It's Scared?
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Cats wear their emotions on their tails, not their sleeves. They tap their fluffy rear appendages during relaxing naps, thrash them while tense, and hold them stiff and aloft when they’re feeling aggressive, among other behaviors. And in some scary situations (like, say, being surprised by a cucumber), a cat’s tail will actually expand, puffing up to nearly twice its volume as its owner hisses, arches its back, and flattens its ears. What does a super-sized tail signify, and how does it occur naturally without help from hairspray?

Cats with puffed tails are “basically trying to make themselves look as big as possible, and that’s because they detect a threat in the environment," Dr. Mikel Delgado, a certified cat behavior consultant who studied animal behavior and human-pet relationships as a PhD student at the University of California, Berkeley, tells Mental Floss. The “threat” in question can be as major as an approaching dog or as minor as an unexpected noise. Even if a cat isn't technically in any real danger, it's still biologically wired to spring to the offensive at a moment’s notice, as it's "not quite at the top of the food chain,” Delgado says. And a big tail is reflexive feline body language for “I’m big and scary, and you wouldn't want to mess with me,” she adds.

A cat’s tail puffs when muscles in its skin (where the hair base is) contract in response to hormone signals from the stress/fight or flight system, or sympathetic nervous system. Occasionally, the hairs on a cat’s back will also puff up along with the tail. That said, not all cats swell up when a startling situation strikes. “I’ve seen some cats that seem unflappable, and they never get poofed up,” Delgado says. “My cats get puffed up pretty easily.”

In addition to cats, other animals also experience piloerection, as this phenomenon is technically called. For example, “some birds puff up when they're encountering an enemy or a threat,” Delgado says. “I think it is a universal response among animals to try to get themselves out of a [potentially dangerous] situation. Really, the idea is that you don't have to fight because if you fight, you might lose an ear or you might get an injury that could be fatal. For most animals, they’re trying to figure out how to scare another animal off without actually going fisticuffs.” In other words, hiss softly, but carry a big tail.

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